Tips for House of Grief Battle
What worked for me was to get everyone back into the corridor we came from, causing the stairs and doorway to be a choke point. Then I cast hunger of hadar on that area, forcing them all to try to push through it. I did not worry about trying to attack on the first turn, just tried to GTFO of the open room. Teleport spells like misty step are a big help with that.
The darkness they would cast was still a PITA, but I focused as much as possible on attacking whoever cast darkness, because it is a concentration spell. If they lose concentration (or just die) the darkness goes away.
I still had to play it several times. There are so many enemies, you need some good initiative rolls in order to not get swarmed before you even have a chance to do anything.